The above circuit will charge any 2-cell * Li-Ion battery pack.
Maximum current is about 650 milliamps. The circuit is designed
for batteries of 900mah or higher. Note this circuit is NOT for
Li-Metal batts (i.e. Duralites). Power source can be a 12v Gell
cell (Power panel), or can be powered by a car's cigarette lighter.
I use an old 12v DC wall transformer (800ma or more.) Radio shack
sells a 12v/1amp wall DC adapter #273-1776 that will work. Supply
does not need to be regulated. In fact my cheap supply outputs
17 volts with no load.

Note: Some people have commented that this
circuit is not "Smart enough" to charge a Li-Ion battery
properly. As long as you don't discharge the battery below about
3.0v per cell, this circuit follows the Panasonic recommendations
exactly. Below 2.9v/cell, the batteries will need to be trickle
charged (0.1 C) until they reach 3.0v/cell. Discharge below
2.3v/Cell will damage the battery. The circuit will not overcharge
the battery. When the battery is fully charged, the current drops
to zero (actually, the leakage current of the battery.) It will
maintain the charged state forever. I have left cells on this
charger for months.

When used in a flight pack with a ESC with a low-voltage cutoff,
it will normally shut the motor off long before the low voltage
limit is reached. And even then, the battery will "Self recover"
back up a bit. Be sure to measure the pack with a voltmeter (no
load) before charging. Trickle charge if needed. You can convert
the above circuit to a trickle charger by using a switch to select
2 R1 values. A 10 ohm, 1/4 watt value in place of R1 will work
as a trickle charger.

Notes:

Do not charge batteries discharged below 2.9v/cell. If this
occurs,use a trickle charge until 3v/cell is reached. (A standard
wall charger for radio RX batteries should work.)

Input voltage can be from 11-18v.

Q1 (Q2) can be any general-purpose NPN transistor.

All resistors are 1/4 watt except for R1, which is 1/2W or
larger.

C1/2 are any 0.1uf capacitors.

The charge current can be monitored by a voltmeter across
R1. Scale: 1Volt = 1 Amp.

The new version includes an LED that will go off when the
current drops below 20ma or so.

The pinout of the LM317 is shown on the schematic. This pinout
is defined by my CAD package and is not the same as the LM317
data sheet. Be sure to wire according to
the schematic pin diagram.

R6 sets the current that the LED turns
off. This is about 10ma. Use 22 ohm for 25ma, 27ohm for 20ma,
33 ohm for 15ma. Currents are approximate as the tolerances in
the transistor and diodes are not exact and may vary a bit with
temperature.

Assembly:

The entire circuit can be constructed on a small perf board
1" square or so. U1 (LM317) must have a heat sink; a small
piece of aluminium will do. There are many heat sinks available.
The size of the heat sink depends on the input voltage and the
battery capacity. Note: the case of U1 is connected to pin 3,
so the heat sink must be isolated from any other parts of the
circuit. An insulator (TO-220 type) can be used to isolate the
case from the heat sink if needed (i.e. bolting U1 to the case
as a heat sink.) U1 should not get too hot to touch.

Adjust R4 for 8.4V out with no load.

Parts: Radio Shack part numbers given. Improved version added
parts in brackets ().

Description

RS part #

U1

LM317T adj. regulator

276-1778

Q1 (Q2)

2N2222A NPN trans.

276-2009

R1

1 ohm, 1/2 watt resistor (note 1)

271-131

R2

2200 ohm 1/4 w, 5% res

271-1325

R3 (R5,R7)

470 ohm 1/4 watt, 5% res

271-1317

R4

1K trim Pot

271-280

C1,2

0.1uf/50v ceramic cap

272-135

Heat Sink

276-1368

Mounting hardware (note 2)

276-1373

(DS1)

LED (any will do)

(R6)

47 ohm resistor.

(D1)

Any 1N400x diode.

Notes:

1. RS part is 10 watt, only size they carry in 1 ohm.
2. Required if heat sink is to be electrically isolated (recommended.)

How it works:

Panasonic recommends charging at a constant current of 0.7C
until 4.2V/cell is reached. Then constant voltage (CV) is to be
used until current drops to 0.1C. At that time, the charging should
stop. The circuit follows this recommendation exactly. However
it does not turn off the charge. Testing has shown that the current
drops to almost zero anyway.

The circuit simplifies this by limiting the charge voltage
to 8.4v. When the battery reaches 8.4v, it will no longer draw
current. The charger is also current-limited. Below about 75%
charge, the limit current is reached. After about 80% charge,
the current decays toward 0. At about 95% charge the current drops
to only a few milliamps. In theory, the battery will never finish
the charge, the closer it gets, the less current it draws. If
left on the charger for 2 hours or so it will reach near 100%
charge. But, 95% can be reached in less than an hour in most cases
(assumes discharged to 50% or so.) Panasonic's charge curve for
their 830mah batts using this method is shown below:

U1 and resistors R2, R3 and R4 create a voltage regulator.
This sets the termination voltage. U1 will try to maintain 1.25v
across R3. The voltage divider created by R2-R4 multiply this
voltage to get the desired 8.4v value.

Q1 and R1 set the current limit. The current drawn by the battery
passes through R1. As the voltage across R1 approaches 0.65v,
Q1 begins to turn on. Q1 then draws current from the voltage divider.
This fools U1 into thinking the voltage is too high so it reduces
the output.

C1 and C2 are used to reduce noise and guarantee that U1 does
not become instable. Without them, U1 may oscillate.